Selective removal of material using cathodic sputtering



Sept. 6, 1966 M. P. LEPSELTER 3, 7 8

SELECTIVE REMOVAL OF MATERIAL USING CATHODIC SPUTTERING Filed Feb. 25,1964 2 Sheets-Sheet 1 APGON MINIMUM uunnnnnu [/3 nuuuuuun -T uuunun 4//\/l/E/V7OP By M. F. LEPSEL TER A T TORNEV P 6, 1966 M. P. LEPSELTER3,271,286

SELECTIVE REMOVAL OF MATERIAL USING CATHODIC SPUTTERING Filed Feb. 25,1964 2 Sheets-Sheet 2 O O /32 3 i I United States Patent 3,271,286SELECTIVE REMOVAL 6F MATERHAL USING CATHODIC SPUTTERlNG Martin P.Lepselter, Franklin Park, N.J., assignor to Bell Telephone Laboratories,Incorporated, New York,

N.Y., a corporation of New York Filed Feb. 25, 1964, Ser. No. 347,173 2Claims. (Cl. 204192) This application is a continuation-in-part of myapplication Serial No. 331,168, filed December 17, 1963, wherein atechnique is disclosed for the selective removal of material from thesurface of a semiconductive member by what has been termedback-sputtering.

This invention relates to a method for selective removal of materialfrom a workpiece by cathodic sputtering. In particular, the method ofthis invention is useful for selectively removing material from aworkpiece which cannot be subjected to the high potentials usuallyapplied to the cathode member in the sputtering process.

In connection with the cathode sputtering process, it it known that thesurfaces of materials may be cleaned or, in effect, etched by means ofthe sputtering process inasmuch as the cathode member itself is erodedby the bombardment which is basic to sputtering. It is apparent that aform of cleaning occurs as a consequence of the removal of surfacelayers. However, the application of the cathode sputtering technique toremove layers from the surface of semiconductor bodies using potentialsof the order of three to ten kilovolts across the cathode member islikely to cause permanent damage to the semiconductor body. This damageis particularly significant in the case of oxide-protected semiconductorbodies inasmuch as high electric fields cause permanent breakdown of thedielectric coatings.

Accordingly, an object of this invention is an arrangement forselectively removing material from the surface of a workpiece withoutdirectly applying high voltages across the workpiece.

More specifically, an object of the invention is a backsputteringtechnique for effecting selective removal of portions of surfacecoatings on a semiconductor surface which includes dielectric oxidefilms.

In accordance with this invention, the workpiece is separated from thecathode member by an insulating layer and the cathode is so shielded asto result in the formation of a glow dischage region around theperiphery of the semiconductor workpiece. Typically, the glow dischargeregion may be in the form of an annular ring or a rectilinear loop orannulus. This glow discharge region tends to spread somewhat withdistance away from the cathode surface and, accordingly, tends to fillup some of the central volume directly over the workpiece.

The result of the formation of a cathode glow discharge in this patternis to cause the gas ions, which are produced within the vacuum chamberof the sputtering apparatus, to move into the glow discharge region,gain energy and move out by the collision process to the space above theworkpiece, and thence by further collision with other ions to impactsubstantially normally upon the surface of the workpiece. As is known,it is this ion impact which apparently products the sputtering actionand material removal from the impacted surface. Thus, the semiconductorworkpiece surface is bombarded Without applying directly across thesemiconductor member the high voltage field which exists across thecathode member for producing the glow discharge region.

Inasmuch as the ions impact substantially normally on the workpiecesurface, the material removal tends to occur in a very precise mannerwith a practically straightsided cutting action along the boundaries ofthe differentially Ice coated surface. In this connection, the inventionis applicable to any differentially coated surface. By this is meant asurface which either has layers of differing thickness of the samematerial over different portions of the surface or in which differentportions of the surface have coatings of different materials each havingdifferent sputtering efficiencies. In the former case where the samematerial is used but with heavier layers over some portions, thematerial will be removed by sputtering to a substantially equal depthover the entire surface but will, of course, disappear completely fromthe more thinly coated areas first. In the latter case where differentmaterials are used, one material which sputters less efiiciently, andthus more slowly, than the others may constitute a mask to preserveportions underlying the mask while other portions are eroded andremoved.

The invention and its other objects and features will be more clearlyunderstood from the following detailed explanation taken in connectionwith the drawing in which:

FIG. 1 shows in schematic form apparatus suitable for the practice ofthe invention;

FIG. 2 is a plan view of the cathode member, the workpiece and theinterposed insulating and shielding member,

FIG. 3 is a schematic representation of a portion of the apparatusdepicting outlines in cross section of the glow discharge region inaccordance with this invention; and

FIG. 4 is a partial cross section of the semiconductor workpiece, theunderlying portion of the insulating member and a portion of thesupporting cathode member.

Referring to FIG. 1, there is shown in schematic form a vacuum enclosure10 represented by the broken line 11. Within the chamber and forming thebase thereof is an anode member 12 which is held at ground potential.Shieldingly supported within the enclosure is a cathode member 13 towhich a D.-C. supply is connected for applying relatively high negativepotentials of the order of from three to ten kil-ovolts. schematicallyshown penetrating the base of the chamber are connections 14 forevacuating the chamber by conventional means such as diffusion pumps,and an inlet connection 15 with suitable stop valves 16 and 17. Asrepresented by the inlet arrows, the main supply line may be connectedto a source of argon gas and the auxiliary line controlled by stop valve17 to an oxygen supply for providing protective oxide coatings, as willbe explained more fully hereinafter.

As shown in FIG. 2, centrally placed on the cathode 13 is a similarlyshaped ceramic member 18, and centrally mounted on ceramic member 18 isa semiconductor workpiece 19.

A pattern of deposited metallic electrodes in the form of stripes 20 areshown on the surface of the semiconductor workpiece. This pattern isprimarily for exemplary purposes, and it will be understood that avariety of metal patterns may be used with this invention. Therelationship of these stripes 20 to the other parts of the devicestructure are illustrated in more detail in the partial cross sectionshown in FIG. 4. In FIG. 4 the semiconductor workpiece portion 41contains a series of three conductivity type regions making, forexample, a transist-or. In addition to the original bulk portion of thesilicon 41, there is shown a base region 44 defined by the pn junction45 and an emitter region 46 defined by the second dilfused pn junction47. On the surface of the silicon slice is a layer 48 of silicon oxidethrough which openings 61 and 62 have been made to expose surfaceportions of the base and emitter regions 44 and 46, respectively.

Ohmic contact is made initially to the silicon by depositing platinumover the entire oxide layer and heating the material to about 600degrees centigrade. As disclosed in the aforementioned application ofLepselter, this causes a solid phase reaction akin to alloying betweenthe platinum and the silicon. The platinum on the oxide surface, whichis uncombined, then is readily removed by treating the surface with aquaregia. Solutions stronger than the usual three parts hydrochloric acidto one part nitric acid may be used to hasten the speed of platinumremoval. This technique has the advantage I of leaving the alloyedplatinum unaffected.

Following this a layer 49 of titanium is deposited so as to overlie theoxide layer 48 and to make contact to the alloyed surface regions. Ontop of the titanium layer is a second layer 50 of platinum. Finally, ontop of the platinum layer a thin layer of aluminum is deposited. Bymeans of a photoresist technique the aluminum plating is removed, usinga suitable etchant, except for the stripe electrodes 51 and 52 which arein registration with the openings to the base and emitter regions.Inasmuch as the aluminum layer is relatively thin, of the order ofseveral hundred angstroms, chemical etching produces a precise patterndelineation.

An alternative to the formation of these aluminum stripes is theformation of similar gold stripes which are of much greater thickness.Such gold stripes are readily produced to a precise pattern bydepositing the gold on a photoresist pattern which defines the stripearea as uncoated portions of the surface. The gold deposited stripes maybe built up to considerable thickness so as to provide a differentiallycoated surface. Thus, as described hereinafter, although the aluminumhas poor sputtering efliciency and thus is not attacked substantially,the gold will be sputtered but because of its greater thickness willpersist after all of the platinum and titanium have been removed.

To complete the fabrication of the electrode structure of the device inthe semiconductor workpiece, the first and second metal layers, namelytitanium and platinum, must be removed from the workpiece surface exceptwhere these layers underlie the aluminum stripe electrodes 5-1 and 52.In accordance with this invention, this removal is conveniently done bythe process of backsputtering. This technique is particularly valuablewhere the spacing between the electrodes 51 and 52 is in the order ofmicrons.

As shown in the cross sectional view of FIG. 4, the method of theinvention requires an aluminum cathode number 43 upon which thesemiconductor workpiece is mounted using an intermediate ceramicinsulator 42.

An understanding of the uniqueness of the arrangement in accordance withthis invention may be had from a description of the use of theapparatus. The conditions employed in cathodic sputtering are known (seeVacuum Deposition of Thin Films, L. Holland, I. Wiley & Sons, Inc., NewYork, 1956). In this process a known reactive gas such as, for example,hydrogen or any of the members of the rare gas family such as helium,argon or neon is introduced into the chamber. As set forth in the abovetext by Holland, the cathode sputtering system may be operatedstatically by providing a fixed pressure of gas in the system, ordynamically such that a fixed gas pressure is maintained with a constantflow of gas through the system.

In operation, the apparatus of FIG. 1 is subjected to a pressure ofapproximately ten microns of argon gas, and with the anode 12 at groundpotential a negative voltage of five kilovolts is applied to thecathode. Under these conditions, as shown in FIG. 3, a glow dischargeregion having a cross section substantially as depicted by the brokenoutlines 31 is produced around the periphery of the ceramic member 18.This glow discharge region is annular in form and surrounds theworkpiece on all sides, and extends, to some extent, over the spaceabove the workpiece 19. This glow discharge region 31 representsprimarily the cathode discharge region and is the source of energy forparticles passing through or located within the region. Under theconditions described, argon gas ions are produced Within the vacuumchamber. As these particles move about, they collide and some arepropelled into the glow discharge region where they gain energy and, byfurther collision, travel into the space above the workpiece and fromthence into impact with the workpiece surface. One possible path isdepicted by the arrowed broken line 32 connecting the circlesrepresenting gas particles in FIG. 3. It has been found that with theconfiguration shown, the bulk of the bombarding particles impact theworkpiece surface substantially normally and consequently the cuttingaction of the sputtering process produces straightsided structures asrepresented by the broken lines 53 of FIG. 4. As shown in this samefigure, the masking effect of the aluminum electrodes 51 and 52 combinedwith this sputtering process result in very sharply delineated patternson the semiconductor workpiece surface.

The process described above in connection with this invention is mostadvantageous when compared with ordinary chemical etching techniques inwhich the eroding action proceeds at a differential rate as the etchingdepth increases. This tends to produce either undercutting orslope-sided cross sections rather than precise rectilinear crosssections. The value of the back-sputtering technique is particularlynotable where the unmasked area has a width of the order of one-half milor less. For such configurations, the use of chemical etching to depthsof greater than several thousand angstroms is a practical impossibilityinsofar as applicant is aware.

The effectiveness of the arrangement described herein forback-sputtering material from a semiconductor surface on which aprotective oxide coating has been placed will be recognized from theeffect produced if the high cathode potential is applied across theworkpiece itself. If, for example, the silicon oxide coating is about5000 angstroms thick and the applied voltage is five -kilovolts, both ofwhich are typical values, than the field applied to the dielectric oxideis about 10 volts per centimeter. This assumes that, as is the fact,most of the voltage drop occurs across the oxide layer. Inasmuch as thebreakdown voltage for relatively high quality dielectric oxides is about10 volts per centimeter, the result of such high applied fields tends tobe pin-hole breaks through the oxide coating which, of course,effectively destroy the coating as a protective layer. In accordancewith this invention, such consequences are avoided by the arrangementdescribed above.

The foregoing back-sputtering technique for the selective removal ofmetal may be combined advantageously with known procedures fordepositing coatings by cathode sputtering within the same vacuumchamber. In particular, oxide coatings may be produced on thesemiconductor workpiece surface at any desired stage in the fabricationprocess by admitting controlled amounts of oxygen to the inlet linesusing stop valve 17 as noted hereinbefore. By the reactive sputteringprocess well known in the art, oxide coatings of aluminum, for example,where the cathode member is aluminum may be deposited on thesemiconductor member. Thus, it will be apparent that surfaces may bebuilt up by deposition of metals, removed in who-1e or in part, andcoated with dielectric layers, in succession using the apparatus asdescribed above.

Moreover, it has been found that even though the devices disclosed inaccordance with this invention are not provided with the overlaystructure of metal and oxide layers disclosed in the aforementionedLepselter applicaton, the devices herein have extremely good longtermstability which seems to arise simply as a consequence of the oxidecoating having once been covered with metal. In particular, devices ofthe type disclosed herein where the oxide coating covers p-typeconductivity material and where the two layers of metal have beendeposited on, and then removed from the oxide surface, there appears tobe very complete resistance to the formation of surface channels in thismaterial which generally rendered such devices unusable.

Although the invention has been described in terms of a particularembodiment, it will be understood that other arrangemnts may be made bythose skilled in the art which likewise will fall within the scope andspirit of the invention.

For example, other cathode configurations may be employed, even omittingthe ceramic spacing member so long as the workpiece is electricallyinsulated from the cathode and the glow discharge region produced isperipherally disposed and out of contact with the workpiece.

What is claimed is:

'1. A method for the selective removal of material from the surface of asubstrate having a dielectric coating thereon, comprising effecting aglow discharge in a low pressure gaseous ambient between an anode and acath ode whereby an ionized gaseous plasma is formed in said glowdischarge, electrically isolating an area of the surface of said cathodefacing said anode larger than the References Cited by the ExaminerUNITED STATES PATENTS 2,103,623 12/1937 Kott 204-192 2,702,274 2/1955Law 204-192 2,848,542 7/ 195 8 Callahan 204-298 JOHN H. MACK, PrimaryExaminer.

R. MIHALEK, Assistant Examiner.

1. A METHOD FOR THE SELECTIVE REMOVAL OF MATERIAL FROM THE SURFACE OF ASUBSTRATE HAVING A DIELECTRIC COATING THEREON, COMPRISING EFFECTING AGLOW DISCHARGE IN A LOW PRESSURE GASEOUS AMBIENT BETWEEN AN ANODE AND ACATHODE WHEREBY AN IONIZED GASEOUS PLASMA IS FORMED IN SAID GLOWDISCHARGE, ELECTRICALLY ISOLATING AN AREA OF THE SURFACE OF SAID CATHODEFACING SAID ANODE LARGER THAN THE CROSS SECTIONAL AREA OF SAIDSUBSTRATE, BUT HAVING THE PERIPHERAL PORTIONS OF SAID CATHODE INELECTRICAL COMMUNICATION WITH SAID DISCHARGE, AND MOUNTING SAIDSUBSTRATE BETWEEN SAID ANODE AND SAID ELECTRICALLY ISOLATED AREA OF SAIDCATHODE SO THAT NO VOLTAGE DROP OCCURS THROUGH SAID DIELECTRIC COATINGWHEREBY SAID GLOW DISCHARGE IS RESTRICTED TO THE REGION PERIPHERAL TOSAID SURFACE SO THAT IONS ARE IMPACTED ON SAID SURFACE REMOVING MATERIALTHEREFROM.